The present invention relates to an electronic governor for an internal combustion engine for controlling the rotational speed of the internal combustion engine by controlling a control rack of a fuel injection pump.
In an internal combustion engine, such as a diesel engine where fuel is supplied by a fuel injection pump, the rotational speed (rpm) is controlled by controlling the position of a control rack (fuel injection amount adjustment member).
An example of an electronic governor for controlling the rotational speed by controlling the position of the rack of the fuel injection pump for use in an internal combustion engine is shown in Japanese Patent Application Laying-open No. 171037/1982. In this prior art electronic governor, a rotational speed detection signal obtained from a sensor for detecting the rotational speed of the engine, a rack position detection signal obtained from a sensor detecting the position of the rack of the fuel injection pump, and an accelerator position detection signal obtained from a sensor for detecting the position of the accelrator manipulator are used to calculate the target position of the rack required to obtain the desired rotational speed of the engine as indicated by the accelrator position. A control voltage required for positioning the rack at the target position is generated and is used to drive an actuator for actuating the rack, thereby moving the rack to the target position.
It is desirable to maintain substantially constant the power which is proportional to the rotational speed multiplied by the torque dependent on the fuel supply amount. Accordingly, there is an upper limit to the amount of fuel which can be supplied at any given rotational speed. Generally, in the area above the rotational speed giving the maximum torque, the maximum fuel supply amount is decreased as the engine rotational speed is increased. Since the maximum fuel supply amount is determined for each rotational speed, the maximum rack position is determined for each rotatational speed where a fuel injection pump is used, the maximum rack position being defined as the permissible limit position toward the open position side (greater fuel supply side) of the rack. For this reason, the above-mentioned prior art device calculates, from the rack position detection signal and the rotational speed detection signal, the maximum rack position and controls the rack so that its position does not exceed the maximum rack position.
The engine characteristic obtained with the use of the above-described prior art system is shown in FIG. 1. If the load torque T is increased from zero to the full load torque To, while the accelerator is kept at the position where the engine rotational speed N for the no load (torque T=0) is at Ns, engine rotational speed N is decreased to No (&lt;Ns). This characteristic is called droop characteristic. The rack position reaches the maximum rack position at the maximum load torque. With greater load torque, the engine rotational speed N decreases along the maximum rack position characteristic, i.e., with the rack position being controlled at the maximum rack position.
The above-described governor is not satisfactory for applications where the rotational speed should be kept constant against load variation, because it has a droop characteristic and is not capable of providing constant speed characteristic.
For instance, internal combustion engines used for driving welder generator feeding both the welding load and other loads in general are required to have a characteristic shown in FIG.2. This welder generator has switchable (reconnectible) windings so that it can operate both as a constant-frequency AC generator of 50/60 Hz and a welder drive generator. When it is used as an AC generator of 50/60 Hz, the rotational speed (the output frequency of the generator) should be kept constant against variation in the load torque (load of the generator). The range of load variation is from noload to full-load To, and it is required that the engine rotational speed be kept at a constant value No. When the internal combustion engine is used for driving a welder drive generator, the internal combustion engine is operated at or near its full load torque To, and it is required that the rack position be kept at the maximum rack position for the particular rotational speed to produce the maximum output.
As has been described, the conventional electronic governor cannot provide a constant speed characteristic, so that when it is used for driving an AC generator, the output frequency cannot be kept constant.
Another consideration to be made in the design of a governor for an internal combustion engine is the requirement of increasing the fuel supply at the time of starting the engine, i.e., until the starting is completed. For implementing this increase in fuel supply at the time of engine start, it is necessary to detect when the engine is being started, i.e., in a starting condition. In a conventional electronic governor, such a detection is made upon finding of the engine rotational speed below a predetermined rotational speed Nsu which is set below the rotational speed of the starter motor.
A problem encountered by this conventional governor is that if the engine rotational speed becomes low because of increase in the load torque, the starting fuel supply increase control is effected, so that the fuel supply is increased. As a result, the output torque of the engine is increased and hence the rotational speed is increased. When the rotational speed becomes above the predetermined rotational speed Nsu, the fuel supply increase is terminated, so that the output torque again falls and the rotational speed again falls below Nsu. Thus, the unstable operation continues in which the rotational speed fluctuates about Nsu. This may cause over-heating of the engine.